Phase analog encoding system with compensation

ABSTRACT

A phase analog encoding system with compensation for the phase shift error inherent in a resolver position transducer. The inherent phase shift error in a resolver position transducer is compensated independent of a resolver position in a time multiplexed fashion by periodically applying a known reference signal to the resolver and measuring the electrical phase shift across only the resolver. The newly measured value of the inherent electrical phase shift is compared with the last measured value of the inherent electrical phase shift and any deviation is calculated. This deviation is used to determine the appropriate compensation value for the inherent electrical phase shift. The compensation value is subtracted from the resolver phase shift during normal operation resulting in an output signal which is independent of the inherent electrical phase shift error in the resolver position transducer. Additionally, compensation for encoding errors is developed in a calibration mode. This compensation for encoding errors can then be used during the operation of the encoding system.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of co-pending application Ser. No. 827,475 filedon Feb. 5, 1986, now abandoned, which is a continuation of Ser. No.560,658, now abandoned, filed on Dec. 12, 1983.

This application is related to copending U.S. patent application Ser.No. 523,061 filed Aug. 15, 1983, now U.S. Pat. No. 4,577,271 entitled"Sampled Data Servo Control System with Deadband Compensation."

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a phase analog encoding system withcompensation, used in connection with a resolver position transducer andutilized in servo control or monitoring applications.

A resolver position transducer is a device which monitors the positionof a rotatable shaft or a linearly displaceable member by measuring theangular displacement of the shaft or the linear displacement of themember with respect to a fixed reference point. The resolver, whenexcited with the proper electrical input will output an electricalsignal whose phase is related to the position of the shaft or member.Thus the position of the shaft or member is encoded in an electricalsignal in an analog manner. By putting the electrical signal from theresolver position transducer through more encoding circuitry, anelectrical signal can be obtained which represents the position of theshaft or member.

If one knows the position of the shaft or member, one can determine if amachine connected to the shaft or member is operating properly. Thus oneis able to monitor the performance of a machine by measuring the angulardisplacement of a rotatable shaft or the linear displacement of alinearly displaceable member with a resolver position transducer.

Additionally, if one knows the position of a machine's shaft or member,one is able to use that information in a feedback network to control theoperation of the machine in order to obtain any desired performance.Used in this manner, the resolver position transducer is performing aservo-control function.

2. Description of the Prior Art

The phase shift errors inherent in a resolver position transducer are ofparticular importance in applications where the resolver is part of aphase analog encoding system. The phase analog encoding techniqueutilized in such systems involves applying reference signals to theresolver position transducer in the form of two sinusoidal signalsdisplaced in time by 90 electrical degrees such as:

    VR1=K.sub.1 SINωt                                    (1)

    VR2=K.sub.1 COSωt                                    (2)

where VR1 is the voltage across the equivalent of a stator sine winding,VR2 is the voltage across the equivalent of a stator cosine winding andK₁ is a constant. Feedback from the resolver is taken by measuring thevoltage across the equivalent of a resolver rotor winding, VFB. If VR1and VR2 are applied to the equivalent stator sine and cosine windings,the equivalent resolver rotor winding has a voltage of the form:

    VFB=K.sub.2 SIN(ωt+φ+α)                    (3)

where φ is the mechanical displacement of a rotatable shaft or alinearly displaceable member, α is the inherent electrical phase shiftacross the windings of the resolver position transducer, and K₂ is aconstant. If the resolver position transducer is monitoring a rotatableshaft, the mechanical displacement φ is an angular displacement. If theresolver position transducer is monitoring a linearly displaceablemember, the mechanical displacement φ is a linear displacement.

The typical phase analog encoder operates by measuring the relativephase difference (i.e., phase shift) between one of the referencesignals (1) or (2) and the feedback signal (3). This measured phaseshift is equal to the sum of the mechanical displacement φ and an offsetvalue which is the electrical phase shift across the equivalent statorand rotor windings of the resolver φ.

The above encoding technique for measuring the mechanical displacement φwill be accurate as long as α remains constant. Usually α does not varyby more than one or two degress. As a result, the overall phase analogencoding system utilizing this technique is low cost, easy to apply andvery effective for applications where an accuracy of one or two degreesis acceptable.

For many applications, however, the change in α with respect to:temperature; variations in input frequency; manufacturing tolerances andother mechanical constraints such as shaft loading; can be quite large,requiring some form of compensation. Typically, the most severe errorsare introduced by variations in temperature. As a result, some form ofcompensation is necessary where the resolver will be operating in anenvironment with wide variations in the ambient temperature.Compensation is also necessary where the ambient temperature is constantbut the resolver is attached to a device such as a motor which varies intemperature depending on how long the device has been operating.

There are two common forms of compensation for variations intemperature. One form involves mounting a temperature sensor in anetwork to compensate for the inherent electrical phase shift. Thesecond form involves the use of an additional winding in the resolverposition transducer and a separate encoding circuit which is used tomonitor the electrical phase shift across the additional winding so thata compensating signal can be generated which is then used to correct theprimary encoding circuitry of the resolver position transducer.

The disadvantage of both of the common forms of temperature compensationis that they involve additional components and more complex circuitry.This increases the total cost of the system and increase the possibilityof component failure and system break down.

SUMMARY OF THE INVENTION

The present invention overcomes disadvantages and objections associatedwith the prior art compensation for the electrical phase shift acrossthe windings of a resolver position transducer. The disclosed inventionis for an encoding technique wherein the inherent electrical phase shiftacross the windings of a resolver position transducer (windings whichare also part of the primary encoding circuit for determining themechanical displacement of the position transducer) is measured in atime multiplexed fashion to correct for deviations due to all sources,including variations in temperature. It is to be understood that thisinvention can be applied to any sinusoidal position transducer. Wherethe term resolver transducer is used, it is intended to include:synchro, induction potentiometer resolver transmitter, control transfertransformer, differential control transformer and any other sinusoidalposition transducer.

It is to be understood that this invention can be used in connectionwith a rotary resolver position transducer or a linear resolver positiontransducer. In a rotary resolver, the mechanical displacement is anangular displacement and is equal to the rotor angle. In a linearresolver, the mechanical displacement is linear. In a rotary resolver,there are at least two stationary windings called the stator windingsand at least one moveable winding called the rotor winding. A linearresolver has at least two windings which are the electrical equivalentsof the stator windings, and at least one winding which is the electricalequivalent of the rotor winding.

The invention utilizes a resolver position transducer in a phase analogencoding system wherein the total phase shift of the resolver transduceris determined by measuring the time interval between the zero crossingof the resolver since reference and the zero crossing of the resolverfeedback taken from the equivalent of a rotor winding of the resolverposition transducer. The measured time interval between the zerocrossings T is proportional to the sum of the mechanical displacement φand the electrical phase shift across the windings of the resolvertransducer α:

    T=K.sub.3 (φ+α)                                  (4)

where K₃ is the proportionality constant.

Under normal operation (measurement cycle) the two stator windings ofthe resolver transducer are driven by highly accurate sinusoidal signalsdisplaced in time by 90 electrical degrees. If the stator windings areexcited by the above signals, the resolver rotor winding provides aphase analog feedback signal of the form indicated in equation (3). Thetime interval between the zero crossings of two waveforms which have thesame form as equations (1) and (3) is proportional to (φ+α). This timeinterval can be used by a calculating means such as a computer or amicroprocessor to determine (φ+α), φ, and many other useful variables byexecuting predetermined numerical manipulations.

Periodically, at times selected by a calculating means and implementedby a reference switch in the resolver transducer encoding system, theresolver transducer is operated in a compensation mode. During thecompensation mode, the resolver reference voltages (the voltages appliedto the stator windings) are electronically switched and applied to theappropriate resolver stator windings. The appropriate windings aredetermined by the resolver encoding electronics to insure a large signalon the rotor windings. Depending upon the mechanical displacement φ,either a resolver reference signal of K₁ SINωt or -K₁ SINωt is appliedto one or the other of the resolver stator windings. This ensures thatthere will be the maximum possible output on the resolver rotor windingsby applying a resolver reference voltage of the proper sign to thestator winding which has the most magnetic coupling for a givenmechanical displacement φ. For instance, in a rotary resolver positiontransducer where the mechanical displacement φ is an angulardisplacement represented by the mechanical rotor angle:

(a) If the mechanical displacement, φ, is 0°, φ45° or 315°, φ360°, thenK₁ SINωt is applied to the stator cosine winding;

(b) If the mechanical displacement, φ, is 45°, φ135°, then K₁ SINωt isapplied to the stator sine winding;

(c) If the mechanical displacement, φ, is 135°, φ225°, then -K₁ SINωt isapplied to the stator cosine winding;

(d) If the mechanical displacement, φ, is 225°, φ315°, then -K₁ SINωt isapplied to the stator sine winding.

When the resolver reference voltages are applied during the compensationcycle as indicated above, the resolver transducer behaves electricallylike a transformer with the excited stator winding acting like a primarywinding and the rotor winding acting like a secondary winding. As such,the voltage on the secondary winding will only differ in phase from thevoltage on the primary winding by an amount equal to the inherentelectrical phase shift across the resolver windings. Thus, when theresolver encoder measures the time interval between the zero crossing ofthe resolver sine reference and the zero crossing of the resolverfeedback signal, the resulting time interval T is proportional to theinherent electrical phase shift across the windings of the resolvertransducer α:

    T=K.sub.4 (α)                                        (5)

where K₄ is the proportionality constant.

The encoded value of α is then utilized by the calculating means tocorrect for any changes in the inherent electrical phase shift withrespect to previous measurements. Once a proper value for the inherentelectrical phase shift across the resolver windings α is determined, thecalculating means can determine the exact mechanical displacement φ bysubtracting the value of α from the measured quantity obtained duringthe normal measurement cycle, (φ+α).

The duty cycle between the normal measurement cycle (measurement of φ+α)and the compensation cycle (measurement of α only) is determined by thecalculating means and can be either a strict function of time and/or afunction of other variables as deemed appropriate to the application ofthe resolver encoding system.

The phase analog encoding system with compensation for the phase shifterror inherent in a resolver position transducer described by thisinvention has the additional feature of being self-calibrating. Theencoding system can be operated in a calibration mode in which theinherent phase shift error in the encoding circuitry γ is measured. Ifthe inherent phase shift error in the encoding circuitry γ is taken intoaccount, equations (4) and (5), respectively become:

    T=K.sub.3 (φ+α+γ)                          (4A)

    T=K.sub.4 (α+γ)                                (5A)

The calculating means uses the measured value of the inherent phaseshift error in the encoding circuitry γ to compensate for the presenceof this error during the normal measurement mode of operation and thecompensation mode of operation.

The phase analog encoding system described by this inventio can beoperated in a normal measurement mode, a compensation mode or acalibration mode of operation. During the compensation mode ofoperation, the inherent electrical phase shift of the resolvertransducer is measured. This value is used during the normal measurementmode of operation to compensate the measurement of the mechanicaldisplacement φ. During the calibration mode of operation, the inherentphase shift error in the circuitry of the encoding means γ is measured.This measured phase shift error is used to compensate the measurementsmade during the compensation mode and the normal measurement mode ofoperation so that they are independent of any phase shift error inherentin the encoding circuitry.

The inherent phase shift error in the circuitry of the encoding means γis measured in a time multiplexed fashion with the inherent electricalphase shift across the windings of the position transducer α and the sum(φ+α) of the mechanical displacement of the position transducer φ andthe inherent, electrical phase shift across the windings of the positiontransducer α.

Periodically, at times selected by a calculating means and implementedby control electronics, the circuitry of the encoding means is operatedin a calibration mode. During the calibration mode, the signal K₁ SINωtis fed to the encoding circuitry instead of the feedback signal from theresolver transducer. This signal, K₁ SINωt is the same as the referencesignal that is being fed to the encoding circuitry. The measured timeinterval T between the zero crossings of these two signals isproportional to the inherent phase shift error in the encoding circuitryγ:

    T=K.sub.5 (γ)                                        (6)

where K₅ is the proportionality constant.

In an ideal situation, the time period representing the phase shiftbetween an input signal of K₁ SINωt and a reference signal of K₁ SINωtshould be zero. However, in reality, this phase shift may not be zerobecause of an inherent error within the encoding circuitry due to suchthings as the electronic drift of component values over time and changesin temperature. If a value for the inherent phase shift error in theencoding circuitry γ other than zero is measured, this value can beutilized by the calculating means to compensate the measurements madeduring the calibration and normal measurement modes of operation.

The duty cycle between the normal measurement cycle (measurement ofφ+α), the compensation cycle (measurement of α only) and the calibrationcycle (measurement of γ only) is determined by the calculating means andcan be either a strict function of time and/or a function of othervariables deemed to be appropriate to the application of the resolverencoding system. The general theory for the measurement of the inherentphase shift error in the encoding circuitry γ is the same as for themeasurement of the inherent phase shift error across the resolverwindings α except that only K₁ SINωt is needed as a reference signal andcan be used by the encoding means whereas in the measurement of α, K₁SINωt or -K₁ SINωt is applied to either the stator sine winding or thestator cosine winding and the rotor feedback signal is used by theenclding means.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be had to thepreferred embodiment exemplary of the invention shown in theaccompanying drawings, in which:

FIG. 1 shows a schematic of a rotary resolver position transducer withthe reference signals applied to the stator windings and the feedbacksignal taken from the rotor winding in the phase analog form.

FIG. 2 shows a typical phase analog encoding system using a rotaryresolver transducer wherein the relative phase shift between thereference sine winding and the feedback rotor winding is computed by atime interval circuit.

FIG. 3 shows a typical reference frequency generator where the value ofthe divider determines the resolution of the encoding system.

FIG. 4 shows the timing diagram for the encoding of the phase shift inthe time interval.

FIG. 5 shows a block diagram of the resolver based phase analog encodingsystem described by this invention.

FIG. 6 shows the resolver based phase analog encoding system describedby this invention as applied to a rotary resolver position transducer.

FIG. 7 shows the logic utilized in connection with a rotary resolverposition transducer to determine which reference signal polarity andwhich stator winding should be excited during the compensation cycle inorder to guarantee a rotor signal of sufficient magnitude to make avalid reading of the inherent electrical phase shift across the resolverwindings regardless of the rotor position.

FIG. 8 shows an example of the control electronics which implements thecontrol signal from the calculating means to determine when the resolverencoding means operates in the calibration mode.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the preferred embodiment, the invention is used in connection with arotary resolver position transducer. The mechanical displacement φ in arotary resolver position transducer is the mechanical rotor anglerepresenting the angular displacement of the rotor with respect to thestator windings.

Referring to FIG. 1, a rotary resolver transducer 2 is basically anangle transducer and is well known in the art. Generally, a rotaryresolver transducer includes a rotor 5 having one or more sets of spacedapart windings and a stator 6 having two or more sets of spaced apartwindings. These windings are called a rotor winding 10, and statorwindings 20, respectively.

Typically the resolver stator sine winding 21 is excited by a referencesignal 25 of the form K₁ SINωt and the resolver stator cosine winding 22is excited by a reference signal 26 of the form K₁ COSωt. When thestator windings 20 are excited by the above reference signals 25 and 26,the signal 27 on the feedback rotor winding 10 takes the form K₂ SIN(ωt+φ+α) where the mechanical displacement φ is the mechanical rotorangle measuring the position of the rotor 5 with respect to the statorwindings 20 and φ is the inherent electrical phase shift across theresolver transducer 2.

FIG. 2 shows a typical phase analog encoding system consisting of aresolver transducer 2, as shown in FIG. 1, a resolver encoding means anda resolver reference means, described hereinafter. A typical resolverencoding means consists of two zero crossing detectors 41 and 42 and adigital counter 43. The input to the first zero crossing detector 41 isfrom the sine reference signal 25, and the output is connected to thestart switch of the digital counter 43. The input to the second zerocrossing detector 42 is from the feedback rotor winding 10 and theoutput is connected to the stop switch of the digital counter 43.

The encoding system operates by starting the digital counter 43 when thefirst zero crossing detector 41 determines that the sine referencesignal 25 crosses zero voltage. The digital counter 43 then counts thereference frequency 44 until a zero voltage crossing on the feedbackrotor winding 10 is detected by the second zero crossing detector 42.The digital counter 43 is then stopped. The accumulated value 28 in thedigital counter 43 is equal to the sum of the mechanical rotor angle andthe inherent electrical phase shift of the resolver transducer (φ+α).

The resolution of the encoding system is determined by the referencefrequency 44 which the digital counter 43 counts. The referencefrequency 44 is generated by the resolver reference means which alsogenerates the reference signals 25 and 26 which are applied to theresolver stator windings 20.

A typical resolver reference means, as shown in FIG. 2, consists of areference signal generator 51 and two amplifiers 47 and 48 which areused to increase the strength of the reference signals 25 and 26 beforethey are applied to the resolver stator windings 20. The referencesignal generator 51, as shown in FIG. 3, consists of an oscillator 52, adivider 53 and a 90° phase shifter 54.

The oscillator 52, generates the reference frequency 44 that is countedby the digital counter 43. To generate the reference signals 25 and 26which are applied to the resolver stator windings 20, the output of theoscillator 52 is divided by a value N in a divider 53. The value of Ncan be pre-set or can be varied by the calculating means. Thus thereference frequency 44 is equal to N times the frequency of thereference signals 25 or 26 applied to the stator windings 20. The largerthe value of N, the greater the resolution of the encoding system.

The output of divider 53 is fed through amplifier 48 before beingapplied to the stator sine winding 21. To obtain the reference signal 26which is applied to the stator cosine winding 22, the output of divider52 must be fed through a 90° phase shifter 54. This phase shiftedreference signal is then amplified by amplifier 47 before being appliedto the stator cosine winding 22.

The timing diagram for the phase analog encoding system of FIG. 2 isshown in FIG. 4. The reference signal 25 to the stator sine winding 21is depicted by waveform 71. The voltage on the feedback rotor winding 10is depicted by waveform 74. Waveforms 72 and 75 show the output of thefirst zero crossing detector 41 and the second zero crossing detector42, respectively. Waveform 73 is the output 28 of the digital counter43.

When the stator sine reference waveform 71 crosses zero, the first zerocrossing detector 41 is activated as indicated by waveform 72 and thisstarts the digital counter 43 counting. The digital counter 43 continuesto count the reference frequency 44 until it receives a signal to stop.When the feedback rotor winding waveform 74 crosses zero, the secondzero crossing detector 42 is actived, as indicated by waveform 75 andthis sends a stop counting signal to the digital counter 43. The timeinterval that the digital counter 43 has counted is equal to the sum ofthe mechanical rotor angle and the inherent electrical phase shift ofthe resolver transducer (φ+α). Typically the digital counter 43 is resetafter the phase angle number is read in preparation for the nextstart/stop sequence.

The present invention discloses a novel and unique system forcompensating for the inherent electrical phase shift error of a resolverposition transducer in a time multiplexed fashion. Additionally, theinvention has a self-calibrating feature. FIG. 5 shows the overall phaseanalog encoding system with compensation as described by the invention.

The resolver reference means 50 generates reference signals 25 and 26which are fed to the resolver transducer 2 and reference signal 44 whichis used by the digital counter 43 in the resolver encoding means 40.Under normal operation the rotor feedback signal 27 from the resolvertransducer 2 is used by the resolver encoding means 40 along with thestator sine reference signal 25 and the reference signal 44 to measurethe sum of the mechanical displacement φ and the inherent electricalphase shift across the resolver α. The calculating means 80, uses theoutput 28 from the encoding means 40 along with any input controlsignals 84 to generate the reset signal 81 for the digital counter 43and the system output signals 83 such as mechanical displacement,mechanical velocity, etc. The calculating means 80 also determinesthrough control signal 82 which reference signals 25 and 26 of theresolver reference means 50 are applied to the resolver transducer 2.

Control signal 82 determines whether the entire encoding system ismeasuring the sum (φ+α) of the mechanical displacement φ and theelectrical phase shift of the resolver α or just the electrical phaseshift of the resolver α. If control signal 82 has the encoding system inthe normal measurement mode of operation the output 28 of the resolverencoding means 40 is the sum (φ+α) of the mechanical displacement φ andthe inherent electrical phase shift of the resolver α. If the controlsignal 82 has the encoding system in the compensation mode of operationthe output 28 of the resolver encoding means 40 is the inherentelectrical phase shift of the resolver, α.

This measured value of α is then compared with an old value of α by thecalculating means 80 and a new value of α is calculated. The new valueof α is then used by the calculating means 80 along with the output 28of the resolver encoding means 40 under the normal measurement mode ofoperation to obtain a value for the mechanical displacement φ, which isindependent of the inherent electrical phase shift of the resolver α.

The calculating means 80, through control signal 119, also determineswhether the entire encoding system is operating in the calibration modeor in the other modes of operation, i.e., the normal measurement mode orthe compensation mode. As will be described in detail infra, controlsignal 119 takes priority over control signal 82.

FIG. 6 shows the invention depicted by the block diagram in FIG. 5 asapplied to a rotary resolver position transducer. The calculating means80 in the preferred embodiment is a microprocessor unit. However, itdoes not have to be so limited. It can be any type of computer,arithimetic logic unit, or appropriate control circuitry and softwarewhich is capable of: processing the output 28 of the resolver encodingmeans 40; generating a control signal 82 for the resolver referencemeans which determines whether the encoding system is in the normalmeasurement mode of operation or in the compensation mode of operation;and generating a control signal 119 for the resolver encoding meanswhich determines whether the encoding system is in the calibration modeof operation or in the other modes of operation, i.e., the normalmeasurement mode or the compensation mode.

The encoding system cannot be in all three modes of operation at once.It can only be in one mode of operation at a time. That is one of theadvantages of this invention. It utilizes the same physical measuringcircuit in a time sharing fashion to calculate the mechanicaldisplacement φ, the inherent electrical phase shift of the resolvertransducer α, and the inherent phase shift across the circuitry of theencoding means γ.

The duty cycle between the measurement cycle (measurement of φ+α) andthe compensation cycle (measurement of α only) is determined by thesoftware of the microprocessor unit and can be either a strict functionof time and/or a function of other variables deemed to be appropriate,depending on the specific use of the encoding system. If the duty cycleis a strict function of time, the measurement cycle occurs on the orderof 1000 times a second and the compensation cycle occurs on the order ofonce every second.

The duty cycle between the combination of the normal measurement cycle(measurement of φ+α), the compensation cycle (measurement of α only),and the calibration cycle (measurement of γ only) is determined by themicroprocessor and the control circuitry.

Reference switch 87, which implements control signal 82, is shown inFIG. 7. In the normal measurement mode of operation, switch 95 is closedconnecting the sine reference signal 25 to the stator sine winding 21and switch 96 is closed connecting the cosine reference signal 26 to thestator cosine winding 22.

In the compensation mode of operation the reference signal K₁ SINωt or-K₁ SINωt is applied to either the stator sine winding 21 or the statorcosine winding 22 depending upon the position of the resolver rotor 5(i.e., the mechanical rotor angle of the resolver φ). To obtain thereference signal -K₁ SINωt, the reference signal 25, K₁ SINωt, is putthrough a signal inverter 88.

If the mechanical rotor angle of the resolver φ is 0°, φ45° or 315°,φ360°, then only switch 98 is closed and the reference signal K₁ SINωtis applied to the stator cosine winding 22.

If the mechanical rotor angle of the resolver φ is 45°, φ135°, then onlyswitch 95 is closed and the reference signal K₁ SINωt is applied to thestator sine winding 21.

If the mechanical rotor angle of the resolver φ is 135°, φ225°, thenonly switch 99 is closed and the reference signal -K₁ SINωt is appliedto the stator cosine winding 22.

If the mechanical rotor angle of the resolver φ is 225°, φ315°, thenonly switch 97 is closed and the reference signal -K₁ SINωt is appliedto the stator sine winding.

The reason for applying the different reference signal (either K₁ SINωtor -K₁ SINωt) to either the stator sine winding 21 or the stator cosinewinding 22 is to ensure that there is a large signal output on theresolver rotor winding 10 and that the measurement of α will be validindependent of the rotor position φ. The microprocessor unit determineswhat the value of φ is at any given point in time during the normalmeasurement cycle. This value of φ is then encoded into a binary numberwhich determines which switch is closed and correspondingly whichconfiguration of reference voltages is applied to the resolver statorwindings. The table in FIG. 7 shows which binary number corresponds towhich range of values of φ and what the reference switch position willbe for that binary number.

FIG. 8 shows an example of the control electronics 120 necessary toimplement control signal 119. In this case, the control electronics 120is composed of an electronic switch 121. Control electronics 120 can bemore elaborate, although it does not have to be. It can be as simple asa relay that moves electronic switch 121 from pole 125 to 126 when acontrol signal 119 is received.

In the normal, measurement mode of operation, or in the compensationmode of operation, electronic switch 121 is connected to pole 125 suchthat the feedback signal 27 is inputted into the second zero crossingdetector 42 of the resolver encoding means 40. In the calibration modeof operation, the electronic switch 121 is connected to pole 126 suchthat the sine reference signal 25 is inputted into the second zerocrossing detector 42 of the resolver encoding means 40.

In the calibration mode, the resolver encoding means measures theelectrical phase shift between the signal at pole 126, K₁ SINωt, and thereference signal, K₁ SINωt, inputted into the first zero crossingdetector 41. If the resolver encoding means is working perfectly, theoutput 28 of the resolver encoding means 40 will be zero since there isno phase shift between two signals of the form K₁ SINωt. If, however,the output 28 of the resolver encoding means 40 is a value other thanzero, this value can be utilized by the calculating means 80 tocompensate for this measured phase shift error when the encoding systemis operating in the normal, measurement mode of operation or thecompensation mode of operation.

Calculating means 80 sends a control signal 119 to the controlelectronics 120 to determine whether the entire encoding system isoperating in the calibrating mode or in the other modes of operation,i.e., the normal measurement mode or the compensation mode. Controlsignal 119 takes priority over control signal 82 since electronic switch121 must be connected to pole 125 before control signal 82 can have aneffect on the resolver encoding means 40.

While a presently preferred embodiment of the invention has been shownand described, it may be otherwise embodies within the scope of theappended claims.

What is claimed is:
 1. A resolver based phase analog encoding system forindicating position with compensation for an inherent electrical phaseshift across said resolver comprising:(a) a resolver transducer; (b) aresolver reference means electrically connected to said resolvertransducer for generating and applying a plurality of reference signalsto said resolver transducer; (c) a resolver encoding means electricallyconnected to said resolver transducer and said resolver reference meansfor measuring a sum (φ+α) of a mechanical displacement φ and an inherentelectrical phase shift across said resolver transducer α, or an inherentelectrical phase shift across said resolver transducer α independent ofthe mechanical displacement φ, or an inherent electrical phase shift dueto said encoding means γ; and (d) a calculating means electricallyconnected to said resolver encoding means and said resolver referencemeans for compensating said measured sum (φ+α) by said measured inherentelectrical phase shift α to obtain a value of said mechanicaldisplacement φ, said calculating means also compensating for saidinherent electrical phase shift due to said encoding means γ, saidcalculating means controlling said resolver reference means and saidresolver encoding means so that said sum (φ+α), said inherent electricalphase shift across said resolver transducer α, and said inherentelectrical phase shift due to said encoding means γ are measured in atime multiplexed manner; and wherein the calculating means, when thesystem is to measure the inherent electrical phase shift α sends afeedback signal to the resolver reference means to activate a referenceswitch in the resolver reference means thereby applying the correctreference signal in the correct orientation to the resolver transducerto compensate for the current value of the mechanical displacement φthereby ensuring that the measurement of the inherent electrical phaseshift α will be valid independent of the mechanical displacement φ ofthe resolver.
 2. A resolver based phase analog encoding system asdescribed in claim 1 wherein said resolver transducer comprises:(a) astator sine winding positioned on a stator; (b) a stator cosine windingpositioned on said stator; and (c) a rotor winding positioned on arotor, said rotor being situated within said stator.
 3. A resolver basedphase analog encoding system as described in claim 1 wherein saidresolver reference means comprises:(a) a reference signal generator forgenerating a plurality of reference signals; (b) a reference signalswitch electrically connecting the output of said reference signalgenerator to said resolver transducer; and (c) an amplifying meanselectrically located between said reference signal generator and saidresolver transducer for increasing the magnitude of said referencesignals before applying them to said resolver transducer.
 4. A resolverbased phase analog encoding system as described in claim 1 wherein saidresolver transducer comprises: a stator sine winding positioned on astator; a stator cosine winding, positioned on said stator; and a rotorwinding positioned on a rotor, said rotor being siturated within saidstator; and wherein said resolver reference means comprises: a referencesignal generator for generating reference signals; a reference signalswitch electrically connecting the output of said reference signalgenerator to said stator sine and cosine windings such that the positionof said reference switch helps determine whether said resolver encodingmeans measures (φ+α), α or γ; and an amplifying means electricallylocated between said reference signal generator and said resolvertransducer for increasing the magnitude of said reference signals thatare applied to said stator sine and cosine windings.
 5. A resolver basedphase analog encoding system as described in claim 1 wherein saidresolver encoding means comprises:(a) a digital counting means, theoutput of said counting means being proportional to (φ+α), α or γ; (b) azero crossing detector electrically connected to said digital countingmeans for starting said digital counting means, said zero crossingdetector measuring when a reference signal is zero; (c) a second zerocrossing detector electrically connected to said digital counting meansfor stopping said digital counting means, said second zero crossingdetector measuring when either the output of said resolver transducer ora reference signal is zero; and (d) control electronics for electricallyconnecting either the output of said resolver transducer or saidreference signal to said second zero crossing detector.
 6. A resolverbased phase analog encoding system as described in claim 1 wherein saidcalculating means is a computer.
 7. A resolver based phase analogencoding system as described in claim 1 wherein said calculating meansis a microprocessor.
 8. A resolver based phase analog encoding system asdescribed in claim 1 wherein said calculating means is a microprocessor,said microprocessor controlling said resolver reference means and saidresolver encoding means to operate said encoding system in either anormal measurement mode, a compensation mode, or a calibration mode,with α measured during said compensation mode, used to compensate saidsum (φ+α) measured during said normal measurement mode and with γmeasured during said calibration mode used to compensate both said sum(φ+α) and α.
 9. A resolver based phase analog encoding system asdescribed in claim 1 wherein said calculating means is a computer, saidcomputer controlling said resolver reference means and said resolverencoding means to operate said encoding system in either a normalmeasurement mode, a compensation mode, or a calibration mode with αmeasured during said compensation mode used to compensate said sum (φ+α)measured during said normal measurement mode and with γ measured duringsaid calibration mode used to compensate both said sum (φ+α) and α. 10.A resolver based phase analog encoding system for indicating positionwith compensation for an inherent electrical phase shift across saidresolver comprising:(a) a resolver transducer; (b) a resolver referencemeans electrically connected to said resolver transducer for generatingand applying a plurality of reference signals to said resolvertransducer, wherein said resolver reference means comprises: a referencesignal generator for generating reference signals; a reference signalswitch electrically connecting the output of said reference signalgenerator to said resolver transducer such that the position of saidreference switch determines whether a resolver encoding means measures asum (φ+α) of a mechanical displacement φ and an inherent electricalphase shift across said resolver transducer α0 or an inherent electricalphase shift across said resolver transducer α; and an amplifying meanselectrically located between said reference signal generator and saidresolver transducer for increasing the magnitude of said referencesignals that are applied to said resolver transducer; (c) a resolverencoding means electrically connected to said resolver transducer andsaid resolver reference means for measuring said sum (φ+α) of saidmechanical displacement φ and said inherent electrical phase shiftacross said resolver transducer α, or said inherent electrical phaseshift across said resolver transducer α independent of the mechanicaldisplacement φ, or said inherent electrical phase shift due to saidencoding means γ; wherein said resolver encoding means comprises: adigital counting means, the output of said counting means beingproportional to (φ+α), α or γ; a zero crossing detector electricallyconnected to said digital counting means for starting said digitalcounting means, said zero crossing detector measuring when a referencesignal is zero; a second zero crossing detector electrically connectedto said digital counting means for stopping said digital counting means,said second zero crossing detector measuring when either the output ofsaid resolver transducer or a reference signal is zero; and controlelectronics for electrically connecting either the output of saidresolver transducer or said reference signal to said second zerocrossing detector; and (d) a calculating means electrically connected tosaid resolver encoding means and said resolver reference means forcompensating said measured sum (φ+α) by said measured inherentelectrical phase shift α to obtain a value of said mechanicaldisplacement φ, said calculating means also compensating for saidinherent electrical phase shift due to said encoding means γ, saidcalculating means controlling said resolver reference means and saidresolver encoding means so that said sum (φ+α), said inherent electricalphase shift across said resolver transducer α and said inherentelectrical phase shift due to said encoding means γ, are measured in atime multiplexed manner; and wherein the calculating means, when thesystem is to measure the inherent electrical phase shift α sends afeedback signal to the resolver reference means to activate thereference signal switch in the resolver reference means thereby applyingthe correct reference signal from the reference signal generator in thecorrect orientation to the resolver transducer to compensate for thecurrent value of the mechanical displacement φ thereby ensuring that themeasurement of the inherent electrical phase shift α will be validindependent of the mechanical displacement φ of the resolver.
 11. Aresolver based phase analog encoding system as described in claim 10wherein said calculating means is a computer.
 12. A resolver based phaseanalog encoding system as described in claim 10 wherein said calculatingmeans is a microprocessor.
 13. A resolver based phase analog encodingsystem as described in claim 8 wherein said calculating means is amicroprocessor, said microprocessor controlling said reference switchand said control electronics to operate said encoding system in either anormal measurement mode, a compensation mode, or a calibration mode withα measured during said compensation mode used to compensate said sum(φ+α) measured during said normal measurement mode and with γ measuredduring said calibration mode used to compensate both said sum (φ+α) andα.
 14. A resolver based phase analog encoding system as described inclaim 10 wherein said calculating means is a computer, said computercontrolling said reference switch and said control electronics tooperate said encoding system in either a normal measurement mode, acompensation mode, or a calibration mode, with α measured during saidcompensation mode used to compensate said sum (φ+α) measured during saidnormal measurement mode and with γ measured during said calibration modeused to compensate both said sum (φ+α) and α.
 15. A resolver based phaseanalog encoding system for indicating the position of a rotatable shaftwith compensation for an inherent electrical phase shift across saidresolver comprising:(a) a resolver transducer; (b) a resolver referencemeans electrically connected to said resolver transducer for generatingand applying a plurality of reference signals to said resolvertransducer, wherein said resolver reference means comprises: a referencesignal generator for generating reference signals; a reference signalswitch electrically connecting the output of said reference signalgenerator to said resolver transducer such that the position of saidreference switch helps determine whether a resolver encoding meansmeasures a sum (φ+α) of a mechanical rotor angle φ and an inherentelectrical phase shift across said resolver transducer α or an inherentelectrical phase shift across said resolver transducer α; and anamplifying means electrically located between said reference signalgenerator and said resolver transducer for increasing the magnitude ofsaid reference signals that are applied to said resolver transducer; (c)a resolver encoding means electrically connected to said resolvertransducer and said resolver reference means for measuring said sum(φ+α) of said mechanical rotor angle φ and said inherent electricalphase shift across said resolver transducer α, or said inherentelectrical phase shift across said resolver transducer α independent ofthe mechanical rotor angle φ, or said inherent electrical phase shiftdue to said encoding means γ; wherein said resolver encoding meanscomprises: a digital counting means, the output of said counting meansbeing proportional to (φ+α), α or γ; a zero crossing detectorelectrically connected to said digital counting means for starting saiddigital counting means, said zero crossing detector measuring when areference signal is zero; a second zero crossing detector electricallyconnected to said digital counting means for stopping said digitalcounting means, said second zero crossing detector measuring when eitherthe output of said resolver transducer or a reference signal is zero;and control electronics for electrically connecting either the output ofsaid resolver transducer or said reference signal to said second zerocrossing detector; and (d) a calculating means electrically connected tosaid resolver encoding means and said resolver reference means forcompensating said measured sum (φ+α) by said measured inherentelectrical phase shift α to obtain a value of said mechanical rotorangle φ, said calculating means also compensating for said inherentelectrical phase shift due to said encoding means γ, said calculatingmeans controlling said resolver reference means and said resolverencoding means so that said sum (φ+α), said inherent electrical phaseshift across said resolver transducer α and said inherent electricalphase shift due to said encoding means γ, are measured in a timemultiplexed manner; and wherein the calculating means, when the systemis to measure the inherent electrical phase shift α sends a feedbacksignal to the resolver reference means to activate the reference signalswitch in the resolver reference means thereby applying the correctreference signal from the reference signal generator in the correctorientation to the resolver transducer to compensate for the currentvalue of the mechanical displacement φ thereby ensuring that themeasurement of the inherent electrical phase shift α will be validindependent of the rotor angle φ of the resolver.
 16. A resolver basedphase analog encoding system as described in claim 15 wherein saidcalculating means is a computer.
 17. A resolver based phase analogencoding system as described in claim 15 wherein said calculating meansis a microprocessor.
 18. A resolver based phase analog encoding systemas described in claim 15 wherein said calculating means is amicroprocessor, said microprocessor controlling said reference switchand said control electronics to operate said encoding system in either anormal measurement mode, a compensation mode, or a calibration mode withα measured during said compensation mode used to compensate said sum(φ+α) measured during said normal measurement mode and with γ measuredduring said calibration mode used to compensate both said sum (φ+α) andα.
 19. A resolver based phase analog encoding system as described inclaim 15 wherein said calculating means is a computer, said computercontrolling said reference switch and said control electronics tooperate said encoding system in either a normal measurement mode, acompensation mode, or a calibration mode with α measured during saidcompensation mode, used to compensate said sum (φ+α) measured duringsaid normal measurement mode and with γ measured during said calibrationmode used to compensate both said sum (φ+α) and α.